ABSTRACT Due to the varied and numerous events that occur in the spinal cord following injury, it is likely that the most effective approaches to promote recovery following injury will use multiple strategies. Thus, as promising strategies are identified, it is critical to determine what other approaches they can effectively combine with and whether the combinations are complementary, additive, or synergistic. The goal of the proposed research is to determine if a new training strategy targeting adaptive features of walking is beneficial alone or in combination with intraspinal delivery of chondroitinase ABC (Ch'ABC) following spinal cord injury (SCI). Although the potential to recover voluntary locomotor function is much greater with motor incomplete SCI in humans and cats, voluntary adaptive features of locomotion remain impaired. As such, these studies will address and target recovery of clinically important skills for effective home and community ambulation following SCI that are not currently addressed in rehabilitation. Ch'ABC and skill training are two diverse, but likely complementary, approaches. Ch'ABC is used to promote a more permissive substrate for axonal growth within the injured nervous system. Specifically, it cleaves the inhibitory chondroitin sulfate glycoaminoglycan (CS GAG) side chains of a large family of molecules that are known to increase at the site of spinal cord injury. In contrast, evolving basic locomotor training approaches target spared spinal circuitry through activity dependent mechanisms to promote functional recovery. Our guiding hypothesis is that skill-training, designed to target balance and limb trajectories, will enhance spontaneous and Ch'ABC-mediated recovery of adaptations necessary for complex locomotor tasks following SCI. Skill training will target adaptive features of locomotion including step length and height, limb accuracy, and balance that permit negotiation of more complex environments. These studies have evolved from our recently published work in the cat model using Ch'ABC as well as our human locomotor studies. The proposed studies are divided into two sets of experiments. The first set will compare the locomotor recovery (Aim 1a) and assess differences in anatomical substrates likely to contribute to structural plasticity (Aim 1b) between cats that receive no training, basic locomotor training (horizontal runway) and skill- training (multiple runways requiring adaptations in the stepcycle). The second set of experiments will compare the locomotor recovery of three groups of cats: Ch'ABC-only, Ch'ABC + basic locomotor training and Ch'ABC + skill-training (Aim 2a). These studies also will assess structural plasticity (Aim 2b). For assessment of structural plasticity, both retrograde and anterograde tracing approaches will be used. Retrograde tracing will focus on labeling of propriospinal as well as cortico-, rubro- and vesitibulospinal neurons after FluoroGold tracer delivery below the lesion. These systems are of particular interest because they may provide novel intraspinal circuitry at the lesion site or may be associated with various skilled locomotor features in normal cats respectively. Anterograde tracing will focus on trajectories and terminations of short propriospinal connections. Comparisons will be made across groups in all aims. Collectively, these studies will provide important information for evidence-based rehabilitation of motor skills (Aim1a), how two therapeutic approaches (training and intraspinal Ch'ABC) complement each other (Aim 2a) and how these approaches individually and combined may impact segmental and supra-segmental structural changes (Aims 1b and 2b).